Hepatitis C virus (HCV) is a leading cause of chronic hepatitis and hepatocellular carcinoma worldwide and infects more than 1% of the world?s population. Successful vaccine development is pivotal in controlling this global health problem. A system for efficient assembly of HCV structural proteins into HCV-like particles (VLPs) in insect cells has been developed in our laboratory. These noninfectious HCV-like particles have similar morphologic, serologic and biophysical properties as the putative virions isolated from HCV infected humans. In contrast to recombinant subunit vaccines, the viral proteins of HCV-like particles may be presented in a native, virion-like conformation and may therefore be superior in eliciting a protective humoral and cellular immune response. The humoral and cellular immunogenicity of the virus-like particles with or without viral p7 protein was studied. The HCV-LPs were inoculated intraperitoneally without any adjuvant into either BALB/c or AAD (C57BL/6 transgenic for HLA-A2.1) mice. Immunized mice developed high titers of anti-E2 antibodies and virus-specific cellular immune responses including cytotoxic T lymphocytes and T helper responses with gamma interferon production. To evaluate the potential of HCV-LPs as a protective immunogen in a surrogate model, we challenged HCV-LP-immunized mice one month after the last boost with a recombinant vaccinia virus expressing core, E1 and E2 (vvHCV-ST). Mice immunized with virus-like particles were protected from challenge with the recombinant vvHCV.S but not the control virus (vvlacZ). In comparison, DNA immunization with a construct expressing the HCV structural genes resulted in much less protection from HCV-vaccinia challenge. We also evaluated the effects of adjuvant AS01B (monophosphoryl lipid A and QS21) and CpG oligodeoxynucleotides (ODN) 10105 on the immunogenicity of HCV-LPs in mice, and showed that the adjuvants, especially the combination of both, enhanced the immune response with a more TH1 bias. We also tested the HCV-LP and the adjuvants in a nonhuman primate model (baboon) and demonstrated induction of robust and broad humoral and cellular immune responses that can be enhanced by the adjuvants. we further test the immunogenicity of HCV-LPs and the effects of novel adjuvant systems in non-human primates. Three groups of four baboons were immunized with HCV-LPs alone or HCV-LPs plus adjuvant, AS01B (monophosphoryl lipid A and QS21) or the combination of AS01B and CpG oligodeoxynucleotides10105. After four immunizations over a 6 month period, all animals developed HCV-specific humoral and cellular immune responses, in particular, antibodies to HCV structure proteins core and E1/E2, and virus-specific cellular immunity including CD4+ (by enzyme-linked immunospot assay for interferon-g) and CD8+ (by intracellular cytokine staining for interferon-g) responses. In addition, the immunogenicity of HCV-LPs was enhanced by the use of adjuvant AS01B and the combination of AS01B and CpG 10105. The overall HCV-specific immune responses were robust and long-lasting (>8 months). Our results indicate that hepatitis C virus-like particles can induce humoral and cellular immune responses and offers a promising approach to vaccine development. We are currently conduct experiments in chimpanzees to test the effectiveness of the HCV-LP as a vaccine candidate.